Hard Axis Direction Research Articles

Geometry effects on low frequency noise in giant magnetoresistance (GMR) sensors

Microfabricated patterned NiFeCo/Cu GMR multilayers have been studied by varying device sensing area. A bridge sensor configuration was used to measure signal and noise as a function of the device size at room temperature, in a dc magnetic field and with a dc sense current. Wide-band noise from 0.1 Hz to 100 Hz was found to be field dependent for both easy and hard axis directions of applied bias field. The noise showed a broad peak which coincides with the highest signal sensitivity for both easy and hard axis cases in agreement with work on other multilayer systems. The demagnetizing fields cause the peak signal to decrease with stripe width. The signal to noise ratio was found to be proportional to the square root of the device area. Narrow-band fluctuation noise in the presence of an ac drive signal in these GMR devices was found to have a sharp peak near to zero de applied field. This was explained in terms of a spin-flop mechanism.

Asymmetry of domain nucleation and enhanced coercivity in exchange-biased epitaxial NiO/NiFe bilayers

Magnetization reversal processes in epitaxial NiO/NiFe bilayers were studied using the magneto-optic indicator film technique. The influence of dislocations on these processes was determined. Remagnetization parallel to the unidirectional anisotropy axis proceeds by domain nucleation and growth, with nucleation center activity being asymmetric with respect to the applied field sign. Magnetization reversal in the hard axis direction occurs by incoherent rotation. The enhanced coercivity and asymmetric nucleation can be explained by taking into account domain wall behavior in the antiferromagnetic layer.

Magnetic properties of arrays of “holes” in Ni80Fe20films

We have studied the magnetization reversal and magnetoresistance behavior in lithographically defined structures based on a 400 Å Ni80Fe20 film. The structures consist of square arrays of holes with size d in the range from 0.5 to 15 μm fabricated using electron beam lithography and an optimized pattern transfer process. For the field applied along the intrinsic easy axis, a marked increase in the coercive field is observed as the hole size is decreased. This has been attributed to the pinning of the domain walls in the vicinity of the holes. However, for the field applied along the intrinsic hard axis direction, there is a marked increase in the remanence as the hole size is reduced due to the competition between the intrinsic uniaxial anisotropy field and the shape induced magnetic anisotropy field. Unusual magnetoresistance effects are observed as a function of orientation of applied field in submicron structures.

Micromagnetics of NiFe/Ag giant magnetoresistance multilayer thin films (abstract)

Granular NiFe/Ag multilayer film exhibits low saturation field and large giant magnetoresistance (GMR) effect, suited as low-field sensor materials.1 However, observed Barkhausen noise in the GMR measurements in these films have been limiting their practical applications.2 Here, we report a micromagnetic modeling study, focusing on the correlation between the micromagnetic properties and film microstructure. It is found that the spatial randomness of intergranular exchange coupling, likely to occur in the film due to its granular microstructure, can cause Barkhausen jumps in GMR response. Figure 1 shows calculated GMR value of a patterned NiFe/Ag/NiFe trilayer with a size of 1.28 μm×1.11 μm, as a function of external fields, applied along its hard axis direction. The Barkhausen jumps can be clearly identified along the GMR response due to its μm size domain switching. In addition to the spatial nonuniformity, effects of various intergranular and interlayer Heisenberg and non-Heisenberg exchange couplings are systematically studied. Figure 2 shows the calculated GMR response for a patterned planar isotropic trilayer film with an interlayer biquadratic exchange coupling. The triangular shape of the GMR response is the characteristics of the biquadratic interlayer exchange coupling.

Effects of grain isolation on magnetic properties of c-axis uniaxially in-plane oriented barium hexaferrite thin films

Magnetic properties of c-axis uniaxially in-plane aligned Ba-ferrite thin films were studied. Ba-ferrite thin films on [110] a-plane single-crystal sapphire substrates were deposited by low-power RF diode sputtering. Different degrees of grain isolation were achieved via postdeposition annealing at various temperatures in a rapid thermal annealing furnace. In this study, a magnetic evaluation of intergranular interaction was carried out using the well-established remanent magnetization /spl Delta/M curve. The results presented indicate a positive magnetostatic interaction, the strength of which is correlated with grain separation when measured along the magnetic easy axis direction. A predominant negative magnetostatic interaction was observed when measured along the magnetic hard axis direction, the magnitude of which is relatively independent of further grain separation.

Size dependence of the magnetoresistance in submicron FeNi wires

The field-dependent magnetoresistance (MR) characteristic and magnetic hysteresis behavior has been studied in 300–500 Å thickness Ni80Fe20 wires of variable width (w) in the range from 0.2 to 10 μm. As the width of the wire decreases, a marked increase in the easy axis coercive field is seen for fields applied along the wire axis and the form of the MR characteristic is markedly modified for the in-plane perpendicular hard axis direction with a large field-dependent MR response observed for applied field strengths exceeding the edge demagnetizing field. The low field hard axis results are discussed in terms of an inhomogeneous spin configuration across the width of the wires arising from the spatial variation of the demagnetizing field.

Magnetic-field behavior of the spin-density-wave state in (TMTSF)2AsF6.

We report the 16.5 GHz microwave response of single crystal samples of (TMTSF${)}_{2}$${\mathrm{AsF}}_{6}$ at temperatures from 1.7--20 K as a function of external magnetic field. With the field applied along the hard (${\mathit{c}}^{\mathrm{*}}$) axis direction, striking changes are observed below ${\mathit{T}}_{\mathit{c}}$ in the microwave dielectric constant and conductivity, which are similar to (but more clear than) those reported in the ${\mathrm{PF}}_{6}$ salt. Following small changes in the low-temperature dielectric response, we are able to resolve the field dependence of the 3.5 and 1.9 K condensate phase boundaries. For the field applied along the chain direction, no changes are observed. Emphasis is placed upon the analysis of the H-T diagram (H\ensuremath{\parallel}${\mathit{c}}^{\mathrm{*}}$) and the series of subphases within the spin-density-wave manifold.

Composite anisotropy multilayers based on amorphous CoNbZr/SiO 2

We investigated a new type of magnetic thin film, i.e. composite-magnetic anisotropy multilayered film based on the amorhous CoNbZr/SiO 2 system. The B- H hysteresis curves are isotropic and have no easy or hard axis directions. Although this composite anisotropy film has an anisotropy energy K u value of nearly zero, the frequency response of the real part of the permeability is excellent and that of the imaginary part of the permeability is low. In order to clarify the mechanism of the remarkable isotropic permeabilities of these multilayers, we extended our study to an eight-layered composite anisotropy multilayer film. The magnitude of permeability, its frequency response and its resonant frequency vary with the thicknesses of SiO 2 layers. A calculation of the initial permeability taking into account the dipole-dipole interaction, derived from the Landau-Lifschitz equation, indicates that the dipole-dipole interaction supports the uniaxial anisotropy of each layer. The magnetization in such layers, the direction of which shifts by 45° from layer to layer, is more stable than that in the layers without dipole-dipole interaction.

Giant Magnetoresistance in NiFeCo/Cu Multilayers

NiFeCo/Cu multilayer films with NiFeCo buffer layers, prepared by rf magnetron sputtering, show a giant magnetoresistance (MR) effect which oscillates as a function of the Cu thickness. At the second peak of this oscillation, multilayer films have an MR ratio of 12% in fields as low as 50 Oe. The film structure and MR properties are stable with respect to annealing at temperatures up to about 300°C. A field sensitivity of about 0.4%/Oe was obtained over the minor loop of the MR curve, measured using a field in the hard-axis direction. The field sensitivity was almost constant up to a frequency of 1 MHz.

A vector magnetometer using a single Permalloy sensor with dual magnetic feedback

Magnetic feedback fields, applied in both hard and easy axis directions of a Permalloy sensor biased with a sinusoidal hard axis field and offset, pulsed easy axis field, allow the magnitude and direction of both orthogonal components of an external field to be determined. The two outputs from the feedback circuit, corresponding to these two components, are independent of sensitivity variations due to bias or temperature changes. The feedback circuit and sensor is a first order system with a corner frequency to sinusoidal inputs proportional to the hard axis bias frequency.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Computation of switching fields of stacked magnetic hexagonal particles with different heights

Switching fields of magnetic hexagonal particles with different heights, which were stacked in the easy axis direction, were investigated by computer simulation. The particles have easy axes perpendicular to their hexagonal planes. When the field is applied in the easy axis direction, the switching field of the stack is determined mainly by the end particle with the lowest switching field. On the other hand, when the field is applied in the hard axis direction, the switching field of the stack is mainly determined by the total height of the stack. The anisotropy field of a stack is also determined by the total height. The angular dependence of the switching field decreases with increasing number of stacked particles and with the total height of the stack. It also decreases when a small particle is stacked at one end of the stack.

Computer simulation of switching fields and magnetization states of interacting cubic particles: Cases with fields applied parallel to the hard axes

Computer simulation was performed to investigate switching fields and magnetization states of an isolated, two, three, and four interacting cubic particles arranged in three kinds of configurations. The easy axes of the particles are oriented in the same directions, and the fields are applied parallel to the hard axis directions. Switching of magnetic moments occurs at fields near H k in all cases; large decreases (or an increase in one case) in the average magnetizations in the easy axis directions ( z directions), M z, occur at the switching fields. When a high field is applied in the hard axis direction ( x direction) and reduced to zero, the magnetic moments of the particles form 180° walls in the yz plane. The lowest switching field in each case is lower than H k.

Computer simulations of switching fields and magnetization reversal mechanisms of interacting cubic particles: Cases with fields applied parallel to the easy axes

Computer simulations were performed to investigate switching fields and magnetization reversal mechanisms of two, three, and four interacting cubic particles arranged in three configurations. The easy axes of all particles were oriented in the same direction, and fields were applied parallel to the easy axis. When three or four particles are aligned in the hard axis direction, the magnetizations of the outer particles switch at a field lower than the field at which the center particle(s) swith(es). When the particles are aligned in the easy axis direction, the magnetizations of all the particles switch at the same field, but the magnetizations of the end particles switch faster than the magnetization of the center particle(s).

Compensation for Temperature Variation in Magnetostrictive Torque Sensor Using Perpendicularly Magnetizing Coils

A robust torque sensor with rapid response was constructed by forming magnetostrictive layers and mounting coils on a carbon steel shaft. A sensitive and reliable Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">40</sub> Ni <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">56</sub> Cr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> magnetostrictive layer was formed using a low-pressure plasma spray method. A magnetic field perpendicular to the magnetostrictive layer plane is generated by a current flowing through the twin coils to magnetize the steel shaft in the radial direction (the hard axis direction). Linear characteristics of the sensor output with respect to the applied torque (between -40 and +40 N-m) were obtained using the twin coils at exciting frequencies of up to 600 kHz. Variations in the sensor output at zero stress and in the sensitivity to temperature variations of the steel shaft were compensated using newly constructed non-inductive coils to detect the temperature of the λ-layers.

Magnetic Properties of Fe-Co-(Zr, Ta) Films Fabricated in Ar+N2 Plasma

The possibility of realizing soft magnetic properties in 40 wt% Co-Fe alloy films with large magnetostriction constants, prepared by sputtering in Ar+N 2 plasma, is examined. The value of H c along the hard-axis direction of magnetization was about 10 Oe on annealing at 500°C. The soft magnetic properties of these films are determined primarily by lattice strain rather than by a macroscopically induced uniaxial magnetic anisotropy K u . The calculated magnetic anisotropy energy U in individual grains, taking account of the magnetoelastic energy, drastically increases with slight increases in the lattice strain. Therefore, in order to obtain excellent soft magnetic properties, lattice strain relaxation to less than 0.1% is important.

Soft-magnetic multilayers for video recording heads

The effect of inducing a uniaxial anisotropy in microcrystalline Fe/FeCrB multilayers by applying a magnetic field during deposition was studied. Not only a reduction of remanent magnetization, but also an increase of permeability (at 10 MHz) are the result. In the hard axis direction a permeability of 6000 is found for a 10 nm/3 nm Fe/FeCrB multilayer. Calculations show that such high permeabilities are interesting for high-density recording heads without ferrite in the core. The domain structure in the head shows the presence of the uniaxial anisotropy. Large unwanted closure domains, in which rotation permeability is low, are present.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Dynamic switching process of sandwich-structured MR elements

The dynamic behavior of sandwich-structured magnetoresistive (MR) memory elements is studied analytically by solving Gilbert's equation together with the one-dimensional micromagnetic torque equation. The typical element is 2 mu m*20 mu m and consists of two 150-AA thick magnetic layers separated by a 50-AA thick nonmagnetic layer. The easy axis is along the short dimension of the element. The theoretical study shows that wall nucleation is present when the switching is driven by a field applied in the easy-axis direction, whereas switching by a field in the hard-axis direction is accomplished by coherent rotation. The switching speed predicted by this one-dimensional model is in the nanosecond range and increases linearly with the external fields. >

Magnetostriction Constants of High Bs CoZr Amorphous Films

The effect of small amounts of Zr in CoZr films is discussed. It was found that a Zr concentration of 4 at% or less caused the magnetostriction constant to decrease to 1 × 10?6. The magnetostriction constant was measured by making one edge of the film stationary, applying a 20 Oe magnetic field in the easy and hard axis directions and measuring the displacement of the free edges. The magnetorestriction constant of films with a Zr concentration of 4.8 at% and with various film thickness was nearly constant (it was somewhat smaller for thinner films), but after heat treatment this constant decreased without saturating. The value decreased with decreases in the Zr concentration. The magnetostriction constant of CoZr films with a Zr concentration of 3.4 ~ 4 at% decreased to approximately 1 × 10?6.

Magnetic Properties and Domain Observations for CoZr/NiFe Multilayer Films

The magnetic properties and domain structures of amorphous Co 90 Zr 10 / polycrystalline Ni 81 Fe 19 (wt%) multilayer films, candidates for use as the pole pieces of thin-film heads, were investigated. After sputter-forming film samples in a 400 Oe magnetic field, they were annealed with a 480 Oe field either in the easy axis and then the hard axis directions, or rotating relative to the sample. The latter type of annealing resulted in increased permeability and a weakened anisotropy field. Observations of domain structures indicated that the multilayer films have stable and reproducible domains in both magnetized and demagnetized states.

Abstract: Cubic anisotropy measurements of ion-implanted layers on magnetic bubble garnets

Effective cubic anisotropy fields of 100 keV Ne+ implanted layers on (111) garnet films can be directly measured by monitoring the in-plane ac susceptibility or loss while a dc in-plane field is varied along the projection of a cubic hard axis. The effective field is defined by the onset of irreversibility in the in-plane rotation of the magnetization. In the presence of various perpendicular bias fields, the effective field boundary can be plotted for magnetization directions from ∼10° to 90° out-of-plane, with the 90° orientation defining the uniaxial anisotropy field, as reported previously. The effective fields have been measured as a function of temperature for (YSmLuCa)3(FeGe)5O12 films subjected to several implantation dosages and annealing temperatures. The measurement technique is described in an accompanying paper. In a uniform layer, the in-plane effective cubic field should vary as sin ϑM cos2 ϑM, with the magnetization angle given approximately by sin ϑM = Hb/ Hku, where Hb is the applied bias field and Hku is the effective in-plane uniaxial anisotropy field of the layer. The effective-field boundary of a lightly implanted film (8×1013 ions/cm2) behaves generally as expected. The maximum in-plane cubic anisotropy occurs near ϑM = 35°, the direction of a 〈100〉 cubic hard axis and shows relatively little change with annealing, whereas the uniaxial anisotropy field drops rapidly with annealing. The temperature dependence of the uniaxial effective field clearly reflects the variation of the cubic anisotropy. With an implantation dosage of 2×1014/cm2, the cubic hard axes are defined only after annealing. In the as-implanted film, the out-of-plane hard axis directions cannot be located. A heavily implanted film (6×1014 ions/cm2) shows completely anomalous behavior: The cubic hard-axis maximum has totally disappeared, as if the out-of-plane structure has been randomized, although the three-fold symmetry of the in-plane hard directions remains well defined. Annealing has little effect on this behavior, except that after a 600 °C anneal the cubic anisotropy shows a large decrease and the in-plane directions are split off from the original 〈211〉 directions. Similarly, the uniaxial anisotropy gives evidence of a breakup of the layer into regions as described earlier. Long-range diffusion of neon atoms is presumed to be responsible for this behavior. We plan to present a more detailed account of this work at a later date. 1 I. Maartense, C. W. Searle, and H. A. Washburn, J. Appl. Phys. 52, 2361 (1981). 2 I. Maartense, J. Appl. Phys. 53, xxxx (1982).